Method of evaluating semiconductor device and apparatus for evaluating semiconductor device

ABSTRACT

A method of evaluating a semiconductor device having an insulated gate formed of a metal-oxide film semiconductor. The semiconductor device has a high potential side and a low potential side, and a threshold voltage that is a minimum voltage for forming a conducting path between the high and low potential sides. The method includes determining a variation of the threshold voltage at turn-on of the semiconductor device by continuously applying an alternating current (AC) voltage to the gate of the semiconductor device, a maximum voltage of the AC voltage being equal to or higher than the threshold voltage of the semiconductor device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2015-204678, filed on Oct. 16,2015, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The embodiments discussed herein are related to a method of evaluating asemiconductor device and an apparatus for evaluating a semiconductordevice.

2. Description of the Related Art

In recent years, in metal oxide semiconductor field effect transistors(MOSFETs), variation of the threshold voltage at the time of turn-onconsequent to the application of voltage to a gate has become a problem.Variation of the threshold voltage leads to problems such as unbalanceof electric current flowing in the semiconductor device (the electriccurrent is not balanced) and decreased current efficiency. Therefore,variation of the threshold voltage at the time of turn-on has to besuppressed and in order to suppress this variation, the thresholdvoltage at the time of turn-on has to be measured accurately.

Among conventional methods of measuring threshold voltage, one proposedmethod involves applying voltage (gate voltage) to a gate for anarbitrary period, and after application of the voltage to the gate issuspended, measuring the threshold voltage by measuring gate voltagedependence of the current flowing between a source and a drain tocalculate the extent to which the threshold voltage varies (for example,see Denais, M., et al, “On-the-fly characterization of NBTI inultra-thin gate oxide PMOSFET's”, IEEE International Electron DevicesMeeting (IEDM) 2004, p. 109-112).

According to another proposed method, alternating current (AC) voltage(gate voltage) such as voltage of a rectangular pulse is applied to agate for an arbitrary period of time, and after application of the ACvoltage to the gate is suspended, the threshold voltage is measured bymeasuring gate voltage dependence of the current flowing between asource and a drain whereby, the extent to which the threshold voltagevaries is calculated (for example, see Japanese Patent ApplicationLaid-Open Publication No. H8-5706).

Nonetheless, with the methods described above, the application ofvoltage to the gate and measurement of the threshold voltage afterapplication of the voltage to the gate is suspended consume a certainamount of time. Therefore, a problem arises in that the impact of theapplication of the voltage to the gate relaxes during a period from thesuspension of the application of the voltage to the gate until themeasurement of the threshold voltage whereby, variation of the thresholdvoltage is underestimated.

Accordingly, a method of measuring the threshold voltage in a statewhere a constant voltage is continuously applied to a gate has beenproposed (for example, see Sometani, Mitsuru, et al, “ExactCharacterization of Threshold Voltage Instability in 4H-SiC MOSFETs byNon-relaxation method”, Materials Science Forum, Vols. 821-823 (2015),pp. 685-688). FIG. 10 is a circuit diagram schematically depicting anapparatus for evaluating a semiconductor device according to aconventional technique described by Sometani, Mitsuru, et al.

The apparatus for evaluating a semiconductor device according to anembodiment depicted in FIG. 10 is an example of an evaluation devicethat measures variation of a threshold voltage V_(th) of a MOSFET 11 toevaluate reliability of the MOSFET 11. The apparatus for evaluating asemiconductor device includes the MOSFET 11, which is an n-channel type,for example, and subject to measurement, and a constant-voltage source12 and a constant-current source 13 that apply electrical stress to theMOSFET 11. A drain of the MOSFET 11 is connected to the constant-currentsource 13, with a source and body of the MOSFET 11 being grounded. Agate of the MOSFET 11 is connected to a positive terminal of theconstant-voltage source 12. A negative terminal of the constant-voltagesource 12 is grounded.

FIG. 11 is a characteristic diagram of the voltage applied to the gateof the MOSFET 11 by the constant-voltage source 12 according to theconventional technique. The constant-voltage source 12 has anelectromotive force equal to or higher than the threshold voltage V_(th)of the MOSFET 11, and continuously applies to the gate of the MOSFET 11,a constant gate voltage V_(g) (≥V_(th)) equal to or higher than thethreshold voltage V_(th) of the MOSFET 11. The amount of variationΔV_(th) of the threshold voltage V_(th) of the MOSFET 11 is found byconverting the amount of variation of a source-drain voltage V_(sd) ofthe MOSFET 11 measured in a state where a source-drain current I_(sd) ofthe MOSFET 11 is maintained to be constant.

SUMMARY OF THE INVENTION

To solve the problems above, according to one aspect of the invention, amethod of evaluating a semiconductor device having an insulated gatestructure formed of a metal oxide film semiconductor includesdetermining variation of a threshold voltage at turn-on of thesemiconductor device, while continuously applying to a gate of thesemiconductor device, AC voltage having a maximum voltage equal to orhigher than the threshold voltage of the semiconductor device.

In the method according, a minimum voltage of the AC voltage is lessthan the threshold voltage of the semiconductor device.

In the method according, in determining the variation: a constantvoltage is applied between a high potential side and a low potentialside of the semiconductor device while the AC voltage is continuouslyapplied to the gate of the semiconductor device, a change of an electriccurrent flowing from the high potential side to the low potential sideof the semiconductor device corresponding to an application time of theAC voltage is measured, and the variation of the threshold voltage atturn-on of the semiconductor device is obtained based on the measuredchange of the electric current.

In the method, the constant voltage is set to be less than a differenceof the maximum voltage and the threshold voltage.

In the method, the variation of the threshold voltage at turn-on of thesemiconductor device is obtained, based on a product of the measuredchange of electric current multiplied by an inverse of a ratio of a timeperiod during which a voltage equal to or higher than the thresholdvoltage of the semiconductor device is applied to an application timeperiod of the AC voltage.

In the method, before the variation is determined, carrier mobility ofthe semiconductor and capacitance of the oxide film are set based on anelectric current flowing in a direction from the high potential sidetoward the low potential side of the semiconductor device and a voltageapplied to the gate of the semiconductor device.

According another aspect of the invention, a device for evaluating asemiconductor device having an insulated gate structure formed of ametal oxide film semiconductor includes a voltage source connected to agate of the semiconductor device and applying to the gate of thesemiconductor device, AC voltage having a maximum voltage equal to orhigher than a threshold voltage of the semiconductor device. An extentto which the threshold voltage varies at turn-on of the semiconductordevice is determined while the AC voltage is continuously applied to thegate of the semiconductor device by the voltage source.

In the device, a minimum voltage of the AC voltage is less than thethreshold voltage of the semiconductor device.

The device further includes a constant-voltage source connected to ahigh potential side of the semiconductor device and applying a constantvoltage between the high potential side and a low potential side of thesemiconductor device. The constant voltage is applied to thesemiconductor device by the constant-voltage source while the AC voltageis continuously applied to the gate of the semiconductor device by thevoltage source. A change of an electric current flowing from the highpotential side to the low potential side of the semiconductor devicecorresponding an application time period of the AC voltage is measured,and the extent to which the threshold voltage varies at turn-on of thesemiconductor device is obtained based on the measured change of theelectric current.

In the device, the constant voltage is set to be less than a differenceof the maximum voltage and the threshold voltage.

In the device, the extent to which the threshold voltage varies atturn-on of the semiconductor device is obtained, based on a product ofthe measured change of the electric current multiplied by an inverse ofa ratio of a time period during which a voltage that is equal to orhigher than the threshold voltage of the semiconductor device is appliedto an application time period of the AC voltage.

In the device, before application of the AC voltage to the gate of thesemiconductor device, carrier mobility of the semiconductor andcapacitance of the oxide film are set based on the electric currentflowing in a direction from a high potential side toward a low potentialside of the semiconductor device and a voltage applied to the gate ofthe semiconductor device.

In the device, the semiconductor device is configured using silicon as asemiconductor material.

In the device, the semiconductor device is configured using siliconcarbide as a semiconductor material.

In the device, the semiconductor device is configured using germanium asa semiconductor material.

In the device, the semiconductor device is configured usingsilicon-germanium as a semiconductor material.

In the device, the semiconductor device is configured using galliumarsenic as a semiconductor material.

In the device, the semiconductor device is configured using galliumnitride as a semiconductor material.

In the device, the semiconductor device is configured using diamond as asemiconductor material.

The device further includes a storage device storing predeterminedinformation. The extent to which the threshold voltage varies at turn-onof the semiconductor device is automatically measured by executing aprogram stored to the storage device in advance.

Objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram schematically depicting an apparatus forevaluating a semiconductor device according to an embodiment;

FIG. 2 is a characteristic diagram depicting temporal variation ofvoltage applied to a gate of a MOSFET 1 by an AC voltage source 2;

FIG. 3 is a cross-sectional view depicting an example of a structure ofa MOSFET, which is subject to measurement by the apparatus forevaluating a semiconductor device according to the present embodiment;

FIG. 4 is a flowchart depicting an outline of a method of evaluating asemiconductor device according to the present embodiment;

FIG. 5 is a characteristic diagram depicting source-drain currentI_(sd)-gate voltage V_(g) characteristics measured by the method ofevaluating a semiconductor device according to the present embodiment;

FIG. 6 is a characteristic diagram depicting temporal variation of asource-drain current I_(sd) measured by the method of evaluating asemiconductor device according to the present embodiment;

FIG. 7 is a characteristic diagram depicting temporal variation ofthreshold voltage V_(th) measured using the method of evaluating asemiconductor device according to the present embodiment;

FIG. 8 is a sectional view depicting an example of a structure of avertical double-diffused MOSFET subject to measurement by the apparatusfor evaluating a semiconductor device according to the presentembodiment;

FIG. 9 is a sectional view depicting an example of a structure of avertical trench MOSFET subject to measurement by the apparatus forevaluating a semiconductor device according to the present embodiment;

FIG. 10 is a circuit diagram schematically depicting an apparatus forevaluating a semiconductor device according to a conventional technique;and

FIG. 11 is a characteristic diagram of a voltage applied to a gate of aMOSFET 11 by a constant-voltage source 12 according to a conventionaltechnique.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of a semiconductor device according to the present inventionwill be described in detail with reference to the accompanying drawings.In the present description and accompanying drawings, layers and regionsprefixed with n or p mean that majority carriers are electrons or holes.Additionally, + or − appended to n or p means that the impurityconcentration is higher or lower, respectively, than layers and regionswithout + or −. In the description of the embodiments below and theaccompanying drawings, identical constituent elements will be given thesame reference numerals and will not be repeatedly described.

An apparatus for evaluating a semiconductor device according to anembodiment is described. FIG. 1 is a circuit diagram schematicallydepicting an apparatus for evaluating a semiconductor device accordingto the present embodiment. The apparatus for evaluating a semiconductordevice depicted in FIG. 1 is an example of an evaluation device thatmeasures the amount of variation of a threshold voltage V_(th) of aMOSFET 1 to evaluate reliability of the MOSFET 1. The apparatus forevaluating a semiconductor device includes the MOSFET 1, which is ann-channel type, for example, and subject to measurement, and an ACvoltage source 2 and a constant-voltage source 3 that apply electricalstress to the MOSFET 1. A drain of the MOSFET 1 is connected to theconstant-voltage source 3, with a source and a body of the MOSFET 1being grounded. A gate of the MOSFET 1 is connected to a positiveterminal of the AC voltage source 2. A negative terminal of the ACvoltage source 2 is grounded.

The AC voltage source 2 has an electromotive force equal to or higherthan the threshold voltage V_(th) of the MOSFET 1, and continuouslyapplies to the gate of the MOSFET 1, an AC voltage (hereinafter, “stressvoltage”) V_(g) such as that depicted in FIG. 2 and, for example, wherethe voltage changes periodically with time and the maximum voltage isequal to or higher than the threshold voltage V_(th) of the MOSFET 1.

FIG. 2 is a characteristic diagram depicting temporal variation of thevoltage applied to the gate of the MOSFET 1 by the AC voltage source 2.The AC voltage source 2 applies periodically to the gate of the MOSFET1, a rectangular voltage having an ON voltage (V_(on)) equal to orhigher than the threshold voltage V_(th) for a certain ON time periodand an OFF voltage (V_(off)) lower than the threshold voltage V_(th) fora certain OFF time period and thereby, turns on/off the MOSFET 1periodically. FIG. 2 depicts an example of application of a rectangularvoltage. The voltage is not limited to a rectangular voltage. Forexample, the AC voltage source 2 may have a sine waveform having amaximum value equal to or higher than the threshold voltage V_(th) and aminimum value lower than the threshold voltage V_(th).

The constant-voltage source 3 applies a constant-voltage stress(source-drain voltage) V_(sd) between the source and the drain of theMOSFET 1 continuously. Accordingly, when the ON voltage is applied tothe gate of the MOSFET 1 by the AC voltage source 2, because the ONvoltage is equal to or higher than the threshold voltage V_(th), theMOSFET 1 is turned on and enters a state where source-drain currentI_(sd) of the MOSFET 1 flows. When the OFF voltage is applied to thegate of the MOSFET 1 by the AC voltage source 2, because the OFF voltageis lower than the threshold voltage V_(th), the MOSFET 1 is turned offand enters a state where the source-drain current I_(sd) of the MOSFET 1does not flow.

The constant-voltage source 3 also functions as a current measurementunit, and when the constant source-drain voltage V_(sd) is applied tothe MOSFET 1, the constant-voltage source 3 continuously measures andmonitors the source-drain current I_(sd) flowing in the MOSFET 1. Thatis, the constant-voltage source 3 measures temporal variation of thesource-drain current I_(sd) flowing in the MOSFET 1, in a state wherethe source-drain voltage V_(sd) of the MOSFET 1 is maintained to beconstant. As the constant-voltage source 3, for example, a so-calledsource measurement unit (SMU) that supplies current or voltage to asubject to be measured and simultaneously measures the voltage appliedto the subject or the current flowing in the subject can be used.

If the ON time period and the OFF time period of the AC voltage source 2are set in units of microseconds (μs), the source measurement unit (SMU)attached to the apparatus for evaluating a semiconductor device cannotfollow changes of the source-drain current I_(sd) due to the change ofthe ON voltage and the OFF voltage. For example, the source-draincurrent I_(sd) measured by the SMU becomes the average current for 1millisecond, which can be followed by the SMU. For example, if the ratiobetween the ON time period and the OFF time period is respectively 50%,the source-drain current I_(sd) becomes 50% of that as compared to whenthe constant voltage is applied. Therefore, by multiplying thesource-drain current I_(sd) by the inverse of the ratio of the ON timeperiod to the voltage application time period (1/(ON time period/(ONtime period+OFF time period))), a source-drain current I^(on) _(sd) whenonly the ON voltage is continuously applied can be obtained.

For example, if the ratio between the ON time period and the OFF timeperiod is respectively 50%, by multiplying the source-drain currentI_(sd) by the inverse of the ratio of the ON time period to the voltageapplication time period (1/(0.5/(0.5+0.5)))=2, the source-drain currentI^(on) _(sd) when only the ON voltage is continuously applied can beobtained.

An amount of variation ΔV_(th) of the threshold voltage V_(th) of theMOSFET 1 is measured by the constant-voltage source 3, and can be foundby converting the amount of variation of the source-drain current I^(on)_(sd) of the MOSFET 1 converted for a case of continuous application ofonly the ON voltage. In particular, the amount of variation ΔV_(th) ofthe threshold voltage V_(th) of the MOSFET 1 is calculated in thefollowing manner. In a boundary condition in which the source-drainvoltage V_(sd) of the MOSFET 1 is sufficiently smaller than thedifference of the ON voltage of the stress voltage V_(g) supplied by theAC voltage source 2 minus the threshold voltage V_(th) of the MOSFET 1(V_(sd)<<ON voltage of V_(g)−V_(th)), the source-drain current I_(sd) ofthe MOSFET 1 is expressed by equation (1).

$\begin{matrix}{I_{sd} = {\frac{Z}{L}\mu_{n}{C_{ox}\left( {V_{g} - V_{th}} \right)}V_{sd}}} & (1)\end{matrix}$

Furthermore, equation (1) is converted into an equation for which thesolution is the threshold voltage V_(th) of the MOSFET 1. Based on thisequation, equation (2) is obtained for converting the amount ofvariation of the source-drain current I_(sd) of the MOSFET 1 from a time(t=0) when the source-drain voltage V_(sd) is applied to the MOSFET 1until a predetermined time t, into the amount of variation ΔV_(th) ofthe threshold voltage V_(th) of the MOSFET 1. L represents a channellength (shortest distance between the source and drain), Z represents achannel width (width of a channel portion in a direction orthogonal tothe channel length), μ_(n) represents carrier mobility, and C_(ox)represents gate insulating film (oxide film) capacitance.

$\begin{matrix}{{\Delta\;{V_{th}(t)}} = \frac{{I_{sd}(t)} - {I_{sd}(0)}}{\frac{Z}{L}\mu_{n}C_{ox}V_{sd}}} & (2)\end{matrix}$

The Z/L×μ_(n)×C_(ox) in the above equation represents a factor requiredwhen the amount of variation of the source-drain current I_(sd) isconverted into the amount of variation ΔV_(th) of the threshold voltageV_(th) (hereinafter, “conversion factor”). The source-drain currentI_(sd) of the MOSFET 1 has a substantially proportional relation withthe stress voltage V_(g) applied to the gate of the MOSFET 1(hereinafter, “I_(sd)-V_(g) characteristics”), and the conversion factor(=Z/L×μ_(n)×C_(ox)) in expression (2) coincides with the slope of theI_(sd)-V_(g) characteristics.

Therefore, by measuring the I_(sd)-V_(g) characteristics before applyingthe constant voltage stress (the source-drain voltage V_(sd) by theconstant-voltage source 3) to the MOSFET 1 and by substituting thesource-drain current I^(on) _(sd) when only the ON voltage iscontinuously applied to the MOSFET 1 for the I_(sd) in equation (2), theamount of variation ΔV_(th) of the threshold voltage V_(th) of theMOSFET 1 can be estimated.

As described above, by converting the amount of variation of thesource-drain current I_(sd) measured by the constant-voltage source 3into the source-drain current I^(on) _(sd) when only the ON voltage iscontinuously applied to the MOSFET 1 and substituting the convertedsource-drain current I^(on) _(sd) for the I_(sd) in equation (2), theamount of variation ΔV_(th) of the threshold voltage V_(th) of theMOSFET 1 can be estimated.

An example of a structure of the MOSFET 1 in which the extent ofvariation of the threshold voltage V_(th) is evaluated by the apparatusfor evaluating a semiconductor device according to the presentembodiment is described. FIG. 3 is a cross-sectional view depicting anexample of the structure of the MOSFET, which is subject to measurementby the apparatus for evaluating a semiconductor device according to thepresent embodiment. In FIG. 3, a lateral MOSFET is depicted as anexample of the structure of the MOSFET 1.

In the MOSFET 1 depicted in FIG. 3, a p-type epitaxial layer 12 being ap-type body region is provided on an n-type semiconductor substrate 11.An n⁺-type source region 13, an n⁺-type drain region 14, and a p⁺-typebody contact region 15 are each provided selectively on a surface layerof the p-type epitaxial layer 12, on an opposite side of the p-typeepitaxial layer 12 with respect to an n-type semiconductor substrate 11side of the p-type epitaxial layer 12.

On the surface of a portion of the p-type epitaxial layer 12 between then⁺-type source region 13 and the n⁺-type drain region 14, a gateelectrode 17 is provided via a gate insulating film 16. A sourceelectrode 18 contacts the n⁺-type source region 13. A drain electrode 19contacts the n⁺-type drain region 14. A body electrode 20 contacts thep⁺-type body contact region 15. The source electrode 18 and the bodyelectrode 20 are grounded.

Although not particularly limited hereto, for example, dimensions andimpurity concentrations of respective parts of the MOSFET 1 may have thefollowing values. The resistivity and the thickness of the n-typesemiconductor substrate 11 are 0.02 Ωcm and 350 μm, respectively. Theimpurity concentration and the thickness of the p-type epitaxial layer12 are 5×10¹⁵/cm³ and 5 μm, respectively. The impurity concentration andthe thickness of the n⁺-type source region 13 are 2×10²⁰/cm³ and 0.3 μm,respectively. The impurity concentration and the thickness of then⁺-type drain region 14 are 2×10²⁰/cm³ and 0.3 μm, respectively. Theimpurity concentration and the thickness of the p⁺-type body contactregion 15 are 2×10²⁰/cm³ and 0.3 μm, respectively. The gate insulatingfilm 16 formed of an oxide film (SiO₂) having a thickness 50 nm.

The method of evaluating a semiconductor device according to the presentembodiment is described taking as an example, a case where the variationdegree of the threshold voltage V_(th) of the MOSFET 1 prepared underthe conditions exemplified above is evaluated. FIG. 4 is a flowchartdepicting an outline of the method of evaluating a semiconductor deviceaccording to the present embodiment.

The source and the body of the MOSFET 1 are grounded first, and thesource-drain current I_(sd) of the MOSFET 1 is measured by sweeping(changing) the gate voltage of the MOSFET 1 in a range from 0 volt to 15volts, in a state where the source-drain voltage V_(sd) is set to aconstant voltage of 0.1 volt, whereby the I_(sd)-V_(g) characteristicsof the MOSFET 1 are obtained (step S1).

The I_(sd)-V_(g) characteristics of the MOSFET 1 at step S1 when themaximum value of the gate voltage V_(g) to be applied to the MOSFET 1 isset to a range from the threshold voltage V_(th) of the MOSFET 1 (=4volts) to the ON voltage of the AC voltage source 2 (=15 volts) aredepicted in FIG. 5. FIG. 5 is a characteristic diagram depicting thesource-drain current I_(sd)-gate voltage V_(g) characteristics measuredby the method of evaluating a semiconductor device according to thepresent embodiment.

The carrier mobility μ_(n) and the gate insulating film capacity C_(ox)of the MOSFET 1 are set based on the I_(sd)-V_(g) characteristics of theMOSFET 1 obtained at step S1 (step S2). In particular, the slope(=Z/L×μ_(n)×C_(ox)) of the I_(sd)-V_(g) characteristics of the MOSFET 1,which is subject to measurement, is set based on the I_(sd)-V_(g)characteristics for a gate voltage V_(g) by which the I_(sd)-V_(g)characteristics have a proportional relation (a straight line), andhigher.

With respect to the I_(sd)-V_(g) characteristics depicted in FIG. 5, theI_(sd)-V_(g) characteristics have a substantially proportional relationat a gate voltage V_(g) of 8 volts and higher. Therefore, the carriermobility μ_(n) and the gate insulating film capacity C_(ox) of theMOSFET 1, that is, the conversion factor (=Z/L×μ_(n)×C_(ox)=5.6×10⁻⁸A/V) in equation (2) is set, based on a portion of the I_(sd)-V_(g)characteristics for gate voltages V_(g) of 8 volts and higher.

The source and the body of the MOSFET 1 are then grounded, and aconstant voltage (the source-drain voltage V_(sd)) of, for example, 0.1volt is applied between the source and the drain of the MOSFET 1 by theconstant-voltage source 3, in a state with the rectangular stressvoltage V_(g) having the ON voltage of 15 volts, the ON time period of10 μs, the OFF voltage of 0 volt, and the OFF time period of 10 μs,being applied to the gate of the MOSFET 1 by the AC voltage source 2.The amount of variation of the source-drain current I_(sd) flowingbetween the source and the drain of the MOSFET 1 is then measured (stepS3).

FIG. 6 depicts temporal variation of the source-drain current I_(sd)measured at step S3, with respect to application time (bias time) of thestress voltage V_(g). FIG. 6 is a characteristic diagram depictingtemporal variation of the source-drain current I_(sd) measured by themethod of evaluating a semiconductor device according to the presentembodiment. As depicted in FIG. 6, the source-drain current I_(sd)varies as the application time of the stress voltage V_(g) increases.

By multiplying the amount of variation of the source-drain currentI_(sd) measured at step S3 by the inverse of the ratio of the ON timeperiod to a voltage application time period (1/(ON time period/(ON timeperiod+OFF time period))), the amount of variation of the source-draincurrent I^(on) _(sd) when only the ON voltage is continuously applied tothe MOSFET 1 can be estimated (step S4).

Under the above conditions, by multiplying the amount of variation ofthe source-drain current I_(sd) measured at step S3 by 1/(10 μs/(10μs+10 μs))=2, the amount of variation of the source-drain current I^(on)_(sd) when only the ON voltage is continuously applied to the MOSFET 1can be estimated.

By converting the amount of variation of the source-drain current I^(on)_(sd) estimated at step S4 into the amount of variation ΔV_(th) of thethreshold voltage V_(th), based on the conversion factor(=Z/L×μ_(n)×C_(ox)=5.6×10⁻⁸ A/V) set at step S2 and the above expression(2) (step S5), evaluation of reliability of the MOSFET 1 is complete.

Thereafter, measures for suppressing variations of the threshold voltageat the time of turn-on of the MOSFET 1 are taken with respect to theMOSFET 1 and circuit units near the MOSFET 1, based on the amount ofvariation ΔV_(th) of the threshold voltage V_(th) obtained at step S5.An example of the measures taken may be increasing the H₂ concentrationwhen post oxidation anneal (POA) treatment is performed after formationof an oxide film or extending the annealing time to suppress variations.

As an example, the amount of variation ΔV_(th) of the threshold voltageV_(th) when the source-drain current I_(sd) has varied from 3.3868×10⁻⁷amperes to 3.3679×10⁻⁷ amperes is 0.0674 volt. The method of evaluatinga semiconductor device according to the embodiment described above iscarried out, for example, by using the apparatus for evaluating asemiconductor device according to the embodiment depicted in FIG. 1.

The temporal variation of the threshold voltage V_(th) of the MOSFET 1measured using the method of evaluating a semiconductor device accordingto the present embodiment is described next. FIG. 7 is a characteristicdiagram depicting temporal variation of the threshold voltage V_(th)measured using the method of evaluating a semiconductor device accordingto the present embodiment. FIG. 7 depicts the temporal variation of thethreshold voltage V_(th) of the MOSFET 1 measured using the method ofevaluating a semiconductor device according to the present embodiment(hereinafter, “disclosed evaluation method”). FIG. 7 also depicts thetemporal variation of the threshold voltage V_(th) of the MOSFET 1measured using, for example, the method of Denais, M., et al describedabove (hereinafter, “conventional evaluation method”) for comparison.

The results depicted in FIG. 7 confirm that the measurement values ofthe threshold voltage V_(th) according to the disclosed evaluationmethod are greater than the measurement values of the threshold voltageV_(th) according to the conventional evaluation method. The reasonthereof is as follows. In the conventional evaluation method, becausethe threshold voltage V_(th) of the MOSFET 1 is measured aftersuspension of the application of the gate voltage to the MOSFET 1, thevariation of the threshold voltage V_(th) relaxes in a period from theapplication of the gate voltage to the MOSFET 1 until the measurement ofthe threshold voltage V_(th), and the value of the threshold voltageV_(th) is underestimated.

On the other hand, in the disclosed evaluation method, because thestress voltage V_(g) is continuously applied to the gate of the MOSFET1, the variation of the threshold voltage V_(th) can be accuratelymeasured without relaxation of the threshold voltage V_(th).

Although the lateral MOSFET depicted in FIG. 3 is described in theembodiment, the method of evaluating a semiconductor device according tothe present invention is applicable to, for example, a verticaldouble-diffused MOSFET (DMOSFET) depicted in FIG. 8 and a verticaltrench MOSFET depicted in FIG. 9.

FIG. 8 is a sectional view depicting an example of a structure of avertical double-diffused MOSFET subject to measurement by the apparatusfor evaluating a semiconductor device according to the presentembodiment. In the vertical double-diffused MOSFET depicted in FIG. 8,an n⁻-type epitaxial layer 82 being an n⁻-type drift region is providedon a surface of an n⁺-type semiconductor substrate 81 being an n⁺-typedrain region. Two p⁺-type regions 83 that are p⁺-type base regions areselectively provided away from each other on a surface layer of then⁻-type epitaxial layer 82, on an opposite side of the n⁻-type epitaxiallayer 82 with respect to an n⁺-type semiconductor substrate 81 side ofthe n⁻-type epitaxial layer 82.

N⁺-type regions 84 being n⁺-type source regions are selectively providedon a surface layer on an opposite side with respect to an n⁺-typesemiconductor substrate 81 side of the two p⁺-type regions 83. A gateelectrode 86 is provided via a gate insulating film 85 on the surface ofa portion the n⁻-type epitaxial layer 82 between the two p⁺-type regions83. Source electrodes 87 contact the p⁺-type region 83 and the n⁺-typeregion 84. A drain electrode 88 is provided on a back surface of then⁺-type semiconductor substrate 81.

Although not particularly limited hereto, for example, dimensions andimpurity concentrations of respective parts of the verticaldouble-diffused MOSFET 1 may have the following values. The resistivityand the thickness of the n⁺-type semiconductor substrate 81 are 0.02 Ωcmand 350 μm, respectively. The impurity concentration and the thicknessof the n⁻-type epitaxial layer 82 are 5×10¹⁶/cm³ and 10 μm,respectively. The impurity concentration and the thickness of thep⁺-type region 83 are 2×10¹⁷/cm³ and 0.5 μm, respectively. The impurityconcentration and the thickness of the n⁺-type region 84 are 2×10²⁰/cm³and 0.3 μm, respectively. The gate insulating film 85 formed of an oxidefilm (SiO₂) having a thickness of 50 nm.

FIG. 9 is a sectional view depicting an example of a structure of thevertical trench MOSFET that is subject to measurement by the apparatusfor evaluating a semiconductor device according to the presentembodiment. In the vertical trench MOSFET depicted in FIG. 9, an n⁻-typeepitaxial layer 92 being an n⁻-type drift region is provided on asurface of an n⁺-type semiconductor substrate 91 being an n⁺-type drainregion. P⁺-type regions 93 being p⁺-type base regions are selectivelyprovided on a surface layer of the n⁻-type epitaxial layer 92, on anopposite side of the n⁻-type epitaxial layer 92 with respect to ann⁺-type semiconductor substrate 91 side of the n⁻-type epitaxial layer92. N⁺-type regions 94 being n⁺-type source regions are selectivelyprovided on a surface layer on an opposite side with respect to ann⁺-type semiconductor substrate 91 side of the p⁺-type regions 93. Atrench structure is formed in the n⁺-type semiconductor substrate 91, onthe side where the n⁻-type epitaxial layer 92 is provided.

A trench 95 penetrates the n⁺-type region 94 and the p⁺-type region 93from the surface of the opposite side of the n⁻-type epitaxial layer 92with respect to the n⁺-type semiconductor substrate 91 side and reachesthe n⁻-type epitaxial layer 92. A gate insulating film 96 is formed onthe bottom and the side wall of the trench 95 along an inner wall of thetrench 95, and a gate electrode 97 is formed on a side of the gateinsulating film 96 in the trench 95. A source electrode 98 contacts thep⁺-type region 93 and the n⁺-type region 94. A drain electrode 99 isprovided on a back surface of the n⁺-type semiconductor substrate 91.

Although not particularly limited hereto, for example, dimensions andimpurity concentrations of respective parts of the vertical trenchMOSFET take the following values. The resistivity and the thickness ofthe n⁺-type semiconductor substrate 91 are 0.02 Ωcm and 350 μm,respectively. The impurity concentration and the thickness of then⁻-type epitaxial layer 92 are 5×10¹⁶/cm³ and 10 μm, respectively. Theimpurity concentration and the thickness of the p⁺-type region 93 are2×10¹⁷/cm³ and 0.5 μm, respectively. The impurity concentration and thethickness of the n⁺-type region 94 are 2×10²⁰/cm³ and 0.3 μm,respectively. The gate insulating film 96 formed of an oxide film (SiO₂)having a thickness of 50 nm.

In the method of evaluating a semiconductor device according to thepresent invention, processes at respective steps can be performedautomatically by executing a prepared program on a computer such as apersonal computer or a workstation. The program is recorded on acomputer-readable recording medium such as a solid state drive (SSD), ahard disk, a flexible disk, a CD-ROM, an MO, or a DVD and is read fromthe recording medium and executed by the computer. The program is atransmission medium that may be distributed via a network such as theInternet.

As described above, according to the present embodiment, by multiplyingthe inverse of the ratio of an ON time period to a voltage applicationtime period and the amount of variation of a source-drain currentmeasured by applying a constant-voltage stress to the drain of theMOSFET with an AC voltage being continuously applied to the gate of theMOSFET, the amount of variation of the source-drain current when onlythe ON voltage is continuously applied can be estimated. By calculatingthe amount of variation of the threshold voltage based on the amount ofvariation of the estimated source-drain current, the source-draincurrent when only the ON voltage is continuously applied can becalculated.

Therefore, even if AC voltage is applied to the gate, the amount ofvariation of the source-drain voltage of the MOSFET can be measured in astate where relaxation of the threshold voltage does not occur at all,and the extent of temporal variation of the threshold voltage can beaccurately evaluated based on the measurement value, withoutunderestimation. Accordingly, unbalance of current flowing in thesemiconductor device (the current is not balanced) and decreased currentefficiency can be suppressed.

The present invention is not limited to the embodiment described above,and various changes can be made without departing from the scope of thepresent invention. For example, in the embodiment described above, acase of using a constant-voltage source having a function of supplyingAC current to a subject to be measured and a function of measuringcurrent applied to the subject has been described. However, the presentinvention is not limited thereto, and the constant-voltage source mayperform only application of a constant voltage to the subject to bemeasured and a current measurement unit that measures the currentapplied to the subject may be newly provided.

In the embodiment described above, the amount of variation of thethreshold voltage is calculated based on the amount of variation of thesource-drain current. However, the present invention is not limitedthereto, and the source-drain voltage may be measured by using, forexample, a source measurement unit in a state where the source-draincurrent of a MOSFET is maintained to be constant, to calculate theamount of variation of the threshold voltage based on the amount ofvariation of the source-drain voltage.

The present invention is applicable to a semiconductor device in whichsilicon (Si), silicon carbide (SiC), germanium (Ge), silicon-germanium(Site), gallium arsenic (GaAs), gallium nitride (GaN), or diamond (C) isused as a semiconductor material. Further, in the embodiment describedabove, the MOSFET has been described as a subject to be measured as anexample. However, the present invention is not limited to the embodimentdescribed above, and a semiconductor device of various structuresincluding a MOS gate (insulated gate formed with a metal-oxide filmsemiconductor) structure can be set as a subject to be measured. Thepresent invention is further applicable when the conductivity type ofrespective regions of the semiconductor device subject to measurement isreversed.

However, when the conventional MOSFET 11 is used as a power device, thevoltage applied to the gate is not always a constant voltage, and ACvoltage of, for example, a rectangular pulse may be applied. Accordingto the method of Sometani, Mitsuru, et al, by continuously applying aconstant voltage to the gate, the impact of relaxation of the thresholdvoltage variation on the measurement of the threshold voltage can beeliminated. However, according to the method of Sometani, Mitsuru, etal, if AC voltage of, for example, a rectangular pulse is applied, theimpact of relaxation of the threshold voltage variation on themeasurement of the threshold voltage becomes difficult to eliminate.According to the method of Denais, M., et al, the threshold voltage ismeasured after application of the AC voltage to the gate is suspended.Therefore, the impact of application of the voltage to the gate isrelaxed, and the variation of the threshold voltage is underestimated.

According to the method of evaluating a semiconductor device and theapparatus for evaluating a semiconductor device of the presentinvention, the extent to which threshold voltage varies can be evaluatedby applying stress voltage to a gate of a semiconductor device whileapplying AC voltage thereto. Therefore, even if AC voltage is applied tothe gate, the amount of variation of the voltage applied between a highpotential side and a low potential side of the semiconductor device canbe measured in a state where relaxation of the threshold voltage doesnot occur at all, and the threshold voltage at the time of turn-on canbe accurately measured based on the measurement value. Accordingly, theextent of temporal variation of the threshold voltage can be accuratelyevaluated and thus, unbalance of current flowing in the semiconductordevice and decreased current efficiency can be suppressed.

As described above, the method of evaluating a semiconductor device andthe apparatus for evaluating a semiconductor device according to thepresent invention are useful for characteristic evaluation of asemiconductor device, and are particularly suitable for evaluating theextent of variation of the threshold voltage at the time of turn-on dueto the application of gate voltage.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A method of evaluating a semiconductor devicehaving an insulated gate formed of a metal-oxide film semiconductor, thesemiconductor device having a high potential side and a low potentialside, and a threshold voltage that is a minimum voltage for forming aconducting path between the high and low potential sides, the methodcomprising: determining a variation of the threshold voltage at turn-onof the semiconductor device by continuously applying an alternatingcurrent (AC) voltage to the gate of the semiconductor device, includingapplying a constant voltage between the high potential side and the lowpotential side of the semiconductor device while continuously applyingthe AC voltage to the gate of the semiconductor device, measuring achange of an electric current flowing from the high potential side tothe low potential side of the semiconductor device in a time periodduring which the AC voltage is applied, and obtaining the variation ofthe threshold voltage at the turn-on of the semiconductor device usingthe measured change of the electric current, a maximum voltage of the ACvoltage being equal to or higher than the threshold voltage of thesemiconductor device, a minimum voltage of the AC voltage being lessthan the threshold voltage of the semiconductor device, and the constantvoltage being set less than a difference between the maximum voltage ofthe AC voltage and the threshold voltage.
 2. The method according toclaim 1, further comprising, before determining the variation, applyinga voltage to the gate of the semiconductor device, and measuring anelectric current flowing from the high potential side to the lowpotential side of the semiconductor device, and setting carrier mobilityof the semiconductor device and capacitance of an oxide film in the gateof the semiconductor device based on the measured electric current andthe voltage applied to the gate of the semiconductor device.
 3. A methodof evaluating a semiconductor device having an insulated gate formed ofa metal-oxide film semiconductor, the semiconductor device having a highpotential side and a low potential side, and a threshold voltage that isa minimum voltage for forming a conducting path between the high and lowpotential sides, the method comprising: determining a variation of thethreshold voltage at turn-on of the semiconductor device by continuouslyapplying an alternating current (AC) voltage to the gate of thesemiconductor device, including applying a constant voltage between thehigh potential side and the low potential side of the semiconductordevice while continuously applying the AC voltage to the gate of thesemiconductor device, measuring a change of an electric current flowingfrom the high potential side to the low potential side of thesemiconductor device in a time period during which the AC voltage isapplied, and obtaining the variation of the threshold voltage at theturn-on of the semiconductor device using the measured change of theelectric current, a maximum voltage of the AC voltage being equal to orhigher than the threshold voltage of the semiconductor device, a minimumvoltage of the AC voltage being less than the threshold voltage of thesemiconductor device, wherein the obtaining the variation of thethreshold voltage includes obtaining the variation of the thresholdvoltage based on a product of the measured change of electric currentand an inverse of an ON time ratio, the ON time ratio being a ratio ofan ON time period, during which the applied AC voltage is equal to orhigher than the threshold voltage of the semiconductor device, to thetime period during which the AC voltage is applied.
 4. A device forevaluating a semiconductor device having an insulated gate formed of ametal-oxide film semiconductor, the semiconductor device having a highpotential side and a low potential side, and a threshold voltage that isa minimum voltage for forming a conducting path between the high and lowpotential sides, the device comprising: a voltage source connected tothe gate of the semiconductor device, the voltage source beingconfigured to continuously apply an alternating current (AC) voltage, amaximum voltage of which is equal to or higher than the thresholdvoltage of the semiconductor device, to the gate of the semiconductordevice at turn-on of the semiconductor device, so as to determine avariation of the threshold voltage at the turn-on of the semiconductordevice, and a constant-voltage source connected to the high potentialside of the semiconductor device, the constant-voltage source beingconfigured to apply a constant voltage between the high and lowpotential sides of the semiconductor device while the voltage sourcecontinuously applies the AC voltage to the gate of the semiconductordevice, and to measure a change of an electric current flowing from thehigh potential side to the low potential side of the semiconductordevice in a time period during which the AC voltage is applied, so thatthe variation of the threshold voltage is determinable using themeasured change of the electric current, wherein the constant-voltagesource is configured to supply, as the constant voltage, a voltage lessthan a difference between the maximum voltage of the AC voltage and thethreshold voltage.
 5. The device according to claim 4, wherein a minimumvoltage of the AC voltage is less than the threshold voltage of thesemiconductor device.
 6. The device according to claim 4, wherein thevariation of the threshold voltage at the turn-on of the semiconductordevice is obtainable using a product of the measured change of theelectric current and an inverse of a ON time ratio, the ON time ratiobeing a ratio of a ON time period, during which the applied AC voltageis equal to or higher than the threshold voltage of the semiconductordevice, to the time period during which the AC voltage is applied. 7.The device according to claim 4, wherein before application of the ACvoltage to the gate of the semiconductor device, carrier mobility of thesemiconductor device and capacitance of an oxide film in the gate of thesemiconductor device are set based on an electric current flowing fromthe high potential side toward the low potential side of thesemiconductor device and a voltage applied to the gate of thesemiconductor device.
 8. The device according to claim 4, wherein thesemiconductor device uses silicon as a semiconductor material thereof.9. The device according to claim 4, wherein the semiconductor deviceuses silicon carbide as a semiconductor material thereof.
 10. The deviceaccording to claim 4, wherein the semiconductor device uses germanium asa semiconductor material thereof.
 11. The device according to claim 4,wherein the semiconductor device uses silicon-germanium as asemiconductor material thereof.
 12. The device according to claim 4,wherein the semiconductor device uses gallium arsenic as a semiconductormaterial thereof.
 13. The device according to claim 4, wherein thesemiconductor device uses gallium nitride as a semiconductor materialthereof.
 14. The device according to claim 4, wherein the semiconductordevice uses diamond as a semiconductor material thereof.
 15. The deviceaccording to claim 4, further comprising a storage device storingprogram instructions, execution of which by a processor causes thevariation of the threshold voltage at turn-on of the semiconductordevice to be automatically determined.